1. Field of the Invention
The present invention relates to a sheet feeding apparatus which feeds a sheet such as an original or a recording sheet to an image forming apparatus such as a printer, a facsimile machine or a copier, and to an image forming apparatus including the sheet feeding apparatus.
2. Description of the Related Art
Some conventional image forming apparatuses such as printers, copiers and facsimile machines have a feeding apparatus which feeds a sheet to the image forming apparatus. In the sheet feeding apparatus, sheets are stacked on a sheet stacking member (middle plate) which is vertically movably provided in a sheet storage portion, the sheets are lifted to a feeding position, and then the sheets are sent out toward the image forming portion by a sheet feeding member.
In such a sheet feeding apparatus, when sheets are to be stacked on a sheet storage portion for adding or exchanging sheets, the sheet storage portion can be pulled out from the sheet feeding apparatus. In association with the pulling-out operation of the sheet storage portion, the sheet stacking member can be lowered to a predetermined sheet stacking position.
The sheet feeding apparatus connected to a printer, for example, is provided with a sheet upper surface detection sensor which detects a height position of the top sheet stacked on the sheet stacking member. Based on the detection information of the sheet upper surface detection sensor, the sheet stacking member moves. With this, the height position of the top sheet is always maintained at a given height.
As the sheet feeding member, if a sheet-feed signal is sent from the image forming apparatus, the sheet-feed roller abuts against the top sheet and rotates, and the top sheet is fed to a next pair of separation rollers.
The pair of separation rollers separately feeds sheets sent by the sheet-feed roller one-sheet at a time, and sends out the sheets to the image forming apparatus. At that time, if the sheet-feed roller sends out the sheet to the pair of separation rollers, the sheet feed roller retracts above the sheet so that the sheet feed roller does not hinder when the pair of separation rollers separate the sheets from each other and does not abut against the sheet. Whenever the sheet-feed signal is sent from the image forming apparatus, the above operation is repeated and sheets are sent out to the image forming apparatus sheet by sheet.
FIGS. 9 and 10 show a conventional elevating mechanism of such a sheet-feed roller 1053. A sheet Sa supported by a middle plate 1101 as a sheet stacking member incorporated in a sheet cassette is sent out by a sheet-feed roller 1053. The sheet-feed roller 1053 is rotatably held by a turning end of a roller holder 1110 which is turnably mounted on a shaft 1109 of the feed roller 1054, and the sheet-feed roller 1053 can move vertically.
The roller holder 1110 is provided with a sensor flag 1110a. The sensor flag 1110a moves to a position where sensor light of the optical sensor 1111 can be blocked. A position where light projection/light shield of the optical sensor 1111 is switched by the sensor flag 1110a is a position at which an appropriate sheet feed pressure is applied to an upper surface of a sheet by the sheet-feed roller 1053 when sheets are fed (position shown in FIG. 9B).
The middle plate 1101 supports a sheet Sa and can be vertically moved by a push-up plate 1102. As shown in FIG. 9C, if the amount of sheets is reduced by feeding sheets and light of the optical sensor 1111 is not blocked by the sensor flag 1110a (light projection state), the middle plate 1101 moves upward. If the optical sensor 1111 is switched from the light projection state by the sensor flag 1110a, the upward movement of the middle plate 1101 is stopped based on this detection. With this, whenever a sheet S stored in the sheet cassette is sent out and the height of the upper surface of the sheets becomes lower than a predetermined level, the middle plate 1101 is moved upward so that the upper surface position of the sheets can be maintained at a predetermined height level where appropriate sheet feeding pressure is applied.
In the conventional sheet feeding apparatus, after a sheet is sent out, the sheet-feed roller 1053 is moved upward and separated from the upper surface of a sheet. This is the same also when the last sheet in the sheet cassette is sent out. Therefore, if the last sheet is sent out, the sheet-feed roller 1053 is moved to its original position above the sheet cassette shown in FIG. 9A.
The sheet-feed roller 1053 is located at the initial position also when sheets are added and the sheet cassette is attached to the apparatus body. When the sheet-feed roller 1053 is in the initial position, the light of the optical sensor 1111 is blocked by the sensor flag 1110a. 
Since the light of the optical sensor 1111 already is blocked when the sheet cassette is to be attached, even if the upper surface of a sheet does not reach the predetermined height range at which an appropriate sheet feeding pressure is applied, a control section determines that the apparatus is in a state where a sheet can be fed. Therefore, when the sheet cassette is attached, the middle plate 1101 can not be moved upward or an upper surface of a sheet can not be moved to the predetermined height range at which the appropriate sheet feeding pressure is applied.
In this case, the sheet feeding operation is started even though a sheet S does not reach a position where a sheet can be fed (“sheet-feeding position”, hereinafter). As a result, there is a fear that a sheet-feeding failure may be caused, or the middle plate 1101 may be brought higher than necessary depending upon control, and inconvenience such as deformation of parts may be caused.
As a method for solving such a problem, there is a conventional technique as shown in FIGS. 10 and 11 (Japanese Patent Application Laid-open No. 2004-043144). According to this technique, there is provided a cam member 1112 which lowers a sheet-feed roller 1053 when a sheet is to be fed, and which brings up the sheet-feed roller 1053 to an initial position above a sheet on standby. When a sheet cassette 1052 is inserted, the cam member 1112 is mechanically rotated through a turning member 1001 in which an abutting member 1052a disposed in the sheet cassette 1052 is disposed in a body, and the lift up of the sheet-feed roller 1053 is released as shown in FIG. 11B. With this, since the sheet-feed roller 1053 is lowered when the sheet cassette 1052 is inserted, a detector can detect an upper surface of a sheet.
However, according to the technique in which the lift up of the sheet-feed roller 1053 is mechanically released when the sheet cassette is inserted, since the lift-up mechanism is required, the cost is increased. The mechanism is constituted such that it works when the cam member 1112 is in a standby position. Therefore, when the cam member 1112 is not in the standby position due to a jam, the lift up of the sheet-feed roller 1053 can not be released even if the sheet cassette 1052 is inserted and as a result, there is a possibility that the detector can not detect.